Oscillators are used in a wide range of systems to provide a reliable reference frequency. Different oscillator designs provide different degrees of stability, typically with cost increasing with the level of stability. Many types of oscillators suffer from temperature dependence and from drift due to aging. This problem is particularly severe with crystal oscillators. In some oscillator designs, the oscillator may be disciplined by determining the frequency offset between the (internal) frequency generated by the oscillator and an external frequency reference. Some Global Navigation Satellite System (GNSS) receivers apply this approach, making use of one or more satellite signal carrier frequencies as the external frequency reference(s). A typical GNSS receiver makes continuous use of an oscillator for operational purposes. The oscillator output is used as a stable frequency reference for down-converting the GNSS signals and for generating the local code replica used to “despread” the spread spectrum baseband signals. Some Global Navigation Satellite System (GNSS) receivers make use of one or more satellite signal carrier frequencies as the external frequency reference(s) to measure and track the frequency offset and allow for this offset in their internal calculations, but do not discipline the oscillator. Other GNSS receivers discipline the oscillator i.e., they use the frequency offset to determine a control voltage and apply this to the crystal oscillator circuit to adjust for temperature and aging effects.
There may be occasions, however, when the external frequency reference is unavailable; the signals from the orbiting satellites may be blocked or suffer from temporary interference. If and when this happens, it is desirable that the receiver goes into ‘holdover’, during which time the receiver attempts to maintain a compensated oscillator frequency based on data collected during the preceding (normal) operating period. Various schemes for providing temperature compensation are known in the prior art.
U.S. Pat. No. 4,922,212 discloses an oscillator temperature-effect compensation scheme that relies upon a predetermined oscillator temperature-frequency transfer curve. An ambient temperature is measured by a temperature sensor and is used to evaluate the transfer curve and to determine a temperature compensation voltage. Aging effects are accounted for using an appropriate value. US2002/0158693 provides a mechanism for predicting the oscillation frequency in terms of physical parameters including temperature. This is then used during periods in which the reference signal is unavailable to adjust the oscillator control voltage.
The temperature characteristics of oscillators are often specific to the individual oscillators. The use of a predefined function is unlikely to be able to take account of individual oscillator behaviour. U.S. Pat. No. 4,746,879 describes an alternative approach which involves creating and storing, during initial device calibration, a look-up table mapping temperatures to compensation values. However, whilst such an approach can take account to some extent of individual device characteristics, it cannot compensate for device aging. U.S. Pat. No. 5,892,408 discloses a related approach.
A better approach involves the use of look-up tables that are dynamically maintained. US2006/0267703 exemplifies this approach. A look-up table is created by periodically measuring and recording ambient temperature and frequency offset/compensation value using the external frequency reference. When the external frequency reference is unavailable, the measured ambient temperature is measured and used to obtain a frequency offset or compensation value from the look-up table. This approach has the advantage that aging effects are inherently compensated for due to the dynamic updating of the look-up table.
U.S. Pat. No. 5,392,005 uses stored compensation values and addresses the aging issue for crystal oscillators by providing a mechanism whereby the temperature compensation factors are adjusted by use of the reference signal when the oscillator frequencies have drifted from the reference signal in excess of a specified tolerance.
The use of dynamically updated look-up tables mapping temperature to offset frequency or compensation value provides an effective mechanism to compensate local crystal oscillators or their outputs for the effects of temperature dependent variations and aging. However, the prior art approaches require the collection and storage of relatively large amounts of data in order to enable compensation over a sufficiently wide range of potential operating temperatures and with sufficient temperature resolution. Especially in the case of mobile receivers, memory requirements present a significant cost and “real estate” overhead.